Coherent coupling of solid-state qubits to microwave photons using circuit quantum electrodynamics (cQED) provides an elegant approach to non-demolition qubit readout and a scalable pathway toward long-range entanglement. Since its monumental success with superconducting qubits, enormous research efforts have been placed on interfacing cQED with electron spin qubits in silicon which have superior lifetimes (T1) on the order of seconds. However, implementing cQED with spin qubits is inherently difficult due to the small magnetic dipole moment of a single electron spin, limiting single spin-photon coupling rates to about 10 Hz.
Here we resolve this decade-long challenge through a series of experiments: First, we demonstrate strong-coupling between a single electron charge qubit in a gate-defined silicon double quantum dot and a microwave photon in a superconducting cavity, enabled by a new device architecture that supports charge coherence times two orders of magnitude longer than previously attained in a semiconductor environment. Combining electric-dipole interaction with spin-charge hybridization in the presence of a magnetic field gradient, we then achieve strong-coupling between a single electron spin and a single microwave photon. Spin-photon coupling rates up to 11 MHz are observed, exceeding direct magnetic-dipole coupling rates by a stunning five orders of magnitude. As an immediate application of strong spin-photon coupling, we demonstrate all-electric control and dispersive readout of the single-spin qubit, laying the foundation for quantum non-demolition readout of semiconductor spin qubits. These results form a critical step toward building a quantum processor with both long lifetimes and high connectivity.
Two further experiments studying valley states in Si quantum dots using cQED are presented in this thesis: We first use the dispersive interaction between the valley states and cavity photons to perform high-resolution measurements of valley splitting. Next, we study hybridized valley-orbit states in Si using Landau-Zener-Stuckelberg-Majorana interferometry. Here we find that valley-orbit mixing significantly reduces the susceptibility of silicon charge qubits to charge noise. These findings not only may accelerate the progress toward controlling valley splitting in Si quantum devices, but also raise the interesting prospect of utilizing the valley degree of freedom to reduce the detrimental impact of charge noise.